The present invention relates to switching of optical signals, and more particularly to an automatic mask trigger for an optical time domain reflectometer that opens an optical switch when an input optical signal exceeds an automatically determined threshold level.
In an optical time domain reflectometer a received optical signal from a fiber optic cable under test is input to an optical detector that converts the received optical signal into an electrical signal. The electrical signal is subsequently sampled, processed, stored and displayed. From the display of the processed electrical signal a picture of the fiber optic cable characteristics is determined, including loss per unit distance, splice losses that cause reflections, and other reflectionless loss locations. Due to the low level of the received optical signal, the sensitivity of the optical detector is relatively great. As a result a large return on the received optical signal may saturate the optical detector so that for a period of time, corresponding to up to four hundred meters of the fiber optic cable length, the electrical signal is saturated and no cable characteristics during that interval can be determined.
To compensate for this response to high level received optical signals a method is described in U.S. Pat. No. 4,769,534 issued Sept. 6, 1988 to Donald L. Brand entitled "Optical Detector with Storage Effects Reduction." An optical switch is interposed between the received optical signal and the optical detector to mask the high level received optical signals from being passed to the optical detector during non-sampling intervals. The optical switch is controlled so that the switch closes only when the received optical signal is to be sampled. The method is automatic, but requires that samples be taken at sufficient intervals that the optical detector is allowed to recover from high level signals.
Another method is manual in nature in that it requires the operator to observe an initial acquisition of data, and then via a cursor or other means to signal the optical switch to turn off at high level times. Thus the high level optical signal is not input to the optical detector, preventing the saturation of the detector and reducing the period when the cable is uncharacterized. However the optical detector is saturated by high level optical returns for the initial data acquisition to determine where the received optical signal has high level pulses that need to be masked.
What is desired is an automatic masking trigger for an optical time domain reflectometer that prevents the saturation of an optical detector and rapidly opens an optical switch only during the time when an optical input signal exceeds a predetermined level to reduce the period of time when a fiber optic cable is uncharacterized.